Viscosity is one of the most important properties of hydraulic fluid in a fluid power system. Viscosity is a measure of the resistance of the fluid to flow, or, in other words, the sluggishness with which the fluid moves. When the viscosity is low, the fluid is thin and has a low body; consequently, the fluid flows easily. Conversely, when the viscosity is high, the fluid is thick in appearance and has a high body; thus, the fluid flows with difficulty.
Maintaining the hydraulic fluid at the ideal viscosity for a given hydraulic system is an important feature of ensuring efficient operation of the system. If the viscosity of the fluid is too high, the system will operate sluggishly and consume greater amounts of power due to this higher resistance to flow. A higher viscosity also tends to inhibit the proper release of entrapped air from the oil. This entrapped air, which causes the oil to appear foamy, tends to reduce the bulk modulus of the oil so that the oil behaves in a "spongy" manner. Utilization of oil having a lower bulk modulus also increases the noise levels of the pump and valves, and decreases the stability of the operation valves and servo control systems. In addition, oil having trapped air can cause premature damage to pumps as a result of cavitation and microscopic burning of the oil as the air bubbles pass from the inlet to the outlet of the pump.
Additionally, high fluid viscosity will result in increased pressure drop through valves and lines. Conversely, too low of a fluid viscosity will result in increased leakage losses past the seals, and excessive wear due to the breakdown of the oil film between moving parts.
Hydraulic fluid, such as oil, becomes thicker, or more viscous, as the temperature decreases, and thinner, or less viscous, when heated. Thus, changes in temperature can have a significant affect on viscosity, and, therefore, efficient operation of the components of the hydraulic system. Further, excessive temperature hastens oxidation of hydraulic oil and causes it to become too thin. This promotes deterioration of seals and packings, and likewise accelerates wear between closely fitting parts of hydraulic components of valves, pumps, and actuators. Conversely, when the fluid is at an optimum temperature, it will exhibit enhanced air release and display a desirable increased fluid bulk modulus. The high fluid bulk modulus, or, in other words, the incompressibility of the fluid at the optimum temperature provides the highly favorable stiffness of hydraulic systems that makes them the frequent choice for many high-power applications.
Changes in the temperature that affect the operation of the hydraulic system may be caused by environmental conditions, or by heat generated in the system itself. Significant sources of heat in the hydraulic loading system include the pump, pressure relief valves, and flow control valves. The operation of the hydraulic system, whether the loading system is operating in a continuous or cyclic mode, may result in undesirable fluid temperature and viscosity characteristics. When the pump is operating in a continuous mode, the constant circulation of fluid, even at low flow rates, may result in an undesirable increase in fluid temperature, or over-temperature condition, and a corresponding decrease in fluid viscosity. Further, when the fluid is stagnant, as between cycles when the pump is operating in the cyclic mode, the fluid may fall below the optimum temperature level, with a corresponding increase in viscosity. Therefore, when the demands of equipment connected to the pump are light, the viscosity will be high, whereas increased usage of the same system results in decreased viscosity.
A common method of maintaining a desired steady-state temperature and, therefore, viscosity of the hydraulic fluid is to use heat exchangers. Heat exchangers are generally in the form of coolers or heaters that may be interposed in the system to increase the heat dissipation rate or the heat generation rate, respectively. Systems using such heat exchangers have a number of disadvantages. Local oil heaters, which usually employ electrical heating elements, tend to be fairly heat intensive and may tend to burn oil. Cooling is generally accomplished with water to oil heat exchangers. If water from this type of cooler leaks into the oil, major problems may result in the hydraulic system.
Further, such heat exchanges are additional components that are generally associated with a dedicated heating or cooling system. As a result, the use of heat exchangers adds to the overall cost and complexity of the hydraulic system, requiring additional hardware, controls, and operator time to monitor, control, and maintain the equipment. Because the use of heat exchangers may be dictated by the environment in which a system will operate, systems are often designed for use in specific applications. For example, heaters rather than coolers are typical in mobile hydraulic equipment that is required to operate in sub-zero temperatures, whereas coolers may be required in a system that operates continuously in a warm environment. Alternately, a hydraulic system that operates intermittently or more heavily at times may require multiple heat exchangers. For example, at times when the system operates infrequently, as when the demands by the connected system are light, the viscosity will be high, consequently requiring heaters to reduce the viscosity to an appropriate level. Conversely, when the demands of the connected system are heavy, the hydraulic system may be used more often, or even continuously, resulting in lower viscosity fluid. In this way, the same system may require coolers to increase the viscosity of the fluid. Thus, the use of heat exchangers contributes to the overall cost, complexity, and physical size of a hydraulic system.